1. Field of the Invention
The present invention is broadly concerned with an improved heat balance sap flow gauge which is especially configured to accommodate irregular, non-circular, and/or very small diameter plant stems and parts and give valid sap flow testing results with such stems or parts. More particularly, the invention is concerned with such a sap flow gauge which is improved by provision of a U-shaped main body carrying a central heating element as well as radial heat flux and temperature differential sensors; in preferred forms, the U-shaped body is provided with filler means disposed between the legs of the U-shaped body for insuring proper engagement between the plant part and the bight heating means. The heating means advantageously is in the form of an elongated resistance heater strip, with one end of the strip being secured to the body and the other end thereof free.
2. Description of the Prior Art
Direct, accurate, non-invasive and continuous measurement and analysis of transpiration in herbaceous plants and trees has long been a goal of agronomists. The heat balance method for measuring plant water loss has provided a reliable technique and has been used on a wide variety of plant species in many diverse applications. Field studies have used sap flow gauges as a means of partitioning evapotranspiration for examining energy transport phenomena in agricultural settings. Other studies have used these gauges to examine water use, the effects of growth retardants on horticultural plants in urban environments and to measure water loss in natural ecosystems.
The theoretical basis for the heat balance method is based upon thermal flow meter techniques used for measuring gas flow rates through a contained volume by applying heat over a short region and measuring the resulting temperature distribution and heat fluxes within the heated segment. Application of this technique to plant stems and a thorough discussion of the mathematical equations are provided by: Sakuratani, Jap. Agricultural Meteorology, 37:9-17 (1981) and Baker et al., Plant, Cell and Environment, 10:777-782 (1987).
Briefly, however, the heat balance method uses energy balance concepts to account for the heat fluxes within an insulated segment of plant stem. The energy balance of a heated plant stem can be defined as EQU Q=Q.sub.v +Q.sub.r +Q.sub.f +S (1)
where Q represents the heat energy supplied, Q.sub.v is the apical and basal heat energy transferred by conduction along the stem axis, Q.sub.r is the radial conduction of energy perpendicular to the stem axis, Q.sub.f is the heat energy transported by the mass flow of water, and S is the rate of change in heat storage of the stem segment, all with units of watts (W). convective energy flux in the sap can be defined as EQU Q.sub.f =cF(T.sub.so -T.sub.si) (2)
where c is the specific heat of water (J/kg.multidot.K), F is the rate of water flow in the stem (kg/s), and T.sub.so -T.sub.si is the temperature difference between the water flowing into and out of the heated segment. S is typically neglected because of the assumption of steady state conditions in the system. By substituting eqn 2 into eqn 1, the formulation of a mass flow equation for the heated stem segment is EQU F=(Q-Q.sub.v -Q.sub.r)/(c(T.sub.so -T.sub.si)) (3)
An important variable in calculating F is the estimate of the gauge conductance, K.sub.g, which is used in determining Q.sub.r. Values for K.sub.g are found by setting F=0 in eqn 3, solving for Q.sub.r, and dividing by the thermopile output. Sap flow rates close to zero can be obtained from excised stems or by using low night-time flow rates and assuming that sap flow is zero.
In most cases, published field studies have used large mature plants with stems larger than 10 mm in diameter and mainly with circular stem radial geometries. One study (Sakuratani, Jap. Agricultural Meteorology, 45:277-280 (1990)), investigated plants with stem diameters smaller than 10 mm, but the species used had a stem with a circular radial geometry, and the gauge design employed did not allow for easy gauge placement and removal. Large stem diameters with circular geometries allowed easy application of typical rigid cylindrical sap flow gauges. Stem diameters of dicot seedlings may be less than 10 mm, while many mature domesticated monocots have stems of 5 mm or less in diameter. Furthermore, stem geometries of many dicot seedlings are not circular, and the geometries of many native monocots are highly elliptical.
One prior sap flow gauge is in the form of a cylindrical member supporting an internal stem heater having resistance heating elements uniformly positioned along the length thereof. The cork backing of the member is composed of three individual pieces of cork, one for the heater and thermopile, and two separate pieces for the upper and lower thermocouple pairs. Different stem sizes are accommodated by altering heater length and width. However, adjusting the heater length for stem diameters near 5 mm or smaller is difficult, because the wire leads needed for input voltage interfere with the stem-heater interface. Also, the provision of three cork pieces makes it difficult for this prior gauge to accommodate non-circular plant stems.
There is accordingly a need in the art for an improved sap flow gauge which can more readily accept small and non-circular stems or plant parts, while at the same time giving accurate stem flow measurement results.